1. Technical Field
The present invention relates to a tube pump More specifically, the present invention relates to a method of driving a tube pump and a liquid ejecting apparatus having the tube pump, which is capable of suppressing any irregular movements of the tube pump as it is switched from a non-operating position to an operating position.
2. Related Art
An ink jet printer is a liquid ejecting apparatus capable of ejecting a liquid onto a target from a liquid ejecting head. During a standard printing process, ink solvent vaporizes from series of openings in the nozzles formed on a nozzle formation surface of a printing or liquid ejecting head. As the solvent vaporizes, the ink solidifies, attracts dust, mixes with air to create bubbles, or the like. For any of these reasons, the nozzles may become clogged, and cause a printing failure. In order to alleviate these problems, the printer generally includes a maintenance unit capable of performing cleaning operation wherein the ink is sucked from the nozzles of the printing head to discharge any solidified ink, dust, or ink bubbles.
Typically, the maintenance unit includes a cam (liquid storage member) which comes in contact with the openings of the nozzles formed on the nozzle formation surface of the printing head and a sucking pump (sucking member) which is provided in an ink discharging passage communicating with the cam. The maintenance unit prevents ink ejection failure by generating a negative pressure in the cam using a sucking pump capable of sucking the ink from the nozzles where the ink with increased viscosity, dust, or air bubbles are formed. An example of one sucking pump currently used in the art is a tube pump disclosed in Japanese Patent Application No. JP-A-2002-349452.
The tube pump disclosed in JP-A-2002-349452 includes a substantially cylindrical housing which houses a midway portion in a flexible tube, a pump foil which rotates about an axis of the housing, and a roller or pressing member which is capable of pressing the tube while moving along an inner circumference of the housing while the pump foil is rotated in a predetermined direction. The midway portion of the tube is housed so as to be enclosed by the inner circumferential surface of the housing. A roller support groove is formed in the pump foil so as to form a curved groove. In addition, the roller support groove is formed so one end of the groove is closer to the shaft center of the pump foil, coinciding with the pump operation position, than on the other end, coinciding with the pump non-operation position. A shaft is inserted in the roller which protrudes from the shaft center and is slid into the roller support groove of the pump foil.
When the pump foil rotates in a pump operating direction, the roller rotates along the edge of the roller support groove. Then, because friction between the roller and the tube is smaller than the friction between the shaft of the roller and edge of the roller support groove, the shaft leaves the an area of the roller support groove associated with the non-operating position Therefore, when the rotation speed of the pump foil is faster than the speed at which the roller shaft slides along the inner circumference of roller support housing toward the pump operation position of the roller support groove.
At the pump operation position, the roller presses the tube to close the tube by causing the inner surfaces of the tube to come in close contact with each other. Accordingly, the roller moves along the inner surface of the housing while pressing a part of the tube against the inner circumference of the housing. When the roller moves, the portion of the tube that was previously pressed against the housing is successively restored to its previous shape, the upstream portion of the tube becomes depressurized, and the ink in the nozzle is sucked into the tube.
One problem with this configuration, however, is that when the roller moves from the pump non-operating position to the pump operating position as the pump foil is rotated toward the pump operating position, due to the difference in friction between the roller and tube and the friction between the roller shaft edge of the roller support groove. That is, the speed that the roller rotates between the pump non-operating position and the pump operating position of the roller depends on the difference between the two frictions. Unfortunately, however, the magnitude of the friction is often irregular due to variations in the environment (such as a temperature or humidity), or the reaction force varying because of irregularity in the tube shape.
Additionally, since the tube pump is mounted in the printer, ink may leak into the housing, lubricating the area between the roller and the tube, causing the friction between the roller and the tube to deteriorate.
In such instances, speed of rotation of the roller along the inner surface of the roller with the rotation is almost equal to the rotation speed of the pump foil. Without a substantial difference in these speeds, the roller shaft does not properly move in the roller support groove. As such, the roller may not adequately move from the pump non-operating position to the pump operating position, meaning that the tube pump may not function as a pump. Thus, it has difficult to successfully design the tube pump so that the difference in friction between roller and the tube and the friction caused by the roller shaft the roller support groove is constant and reliable.